Making PRs close issues

Now that we're making a big push to close out a bunch of issues, I noticed
that for some PRs, someone has to go and manually close out the
corresponding issue.

I think the right way to do this is to include "Fix #123" somewhere in the
merge message that needs to be filled in when you press the merge button.
Then the issue will get automatically closed when the PR is merged.

This is why my merge requests usually have "Fix #123" as the topic; the
merge message includes the topic automatically.

Also, I noticed that github now has a "review" feature (find it on a PR by
clicking the "files changed" button). You can make a comment and press one
of 3 buttons, one of which is "approve".  That seems like a good way to
mark the PR as ready to merge, and is easy to find instead of searching the
conversation for an lgtm. The approvals show up at the bottom, above the
merge request button.